The introduction of a reducing agent into an exhaust gas stream of an exhaust system is considered above all in so-called SCR systems, where SCR denotes Selective Catalytic Reduction. A liquid reducing agent, namely, preferably an aqueous urea solution, is usually considered for use as a reducing agent. The reducing agent evaporates in the exhaust gas stream and can react by thermolysis and subsequent hydrolysis to form ammonia and carbon dioxide. As a result, nitrogen oxides can be reduced into nitrogen and water in an SCR catalytic converter.
An injection device suitable for introducing the reducing agent into the exhaust gas stream usually comprises an exhaust pipe for guiding the exhaust gas stream, which has a connecting piece opening, as well as a connecting piece, which is connected with an inner end to the connecting piece opening and which is provided at an outer end for connecting an injector. The injector can then inject the reducing agent into the exhaust gas stream being led in the exhaust pipe through the connecting piece and through the connecting piece opening.
The tight connection of the connecting piece to the exhaust pipe is comparatively complicated, because the exhaust pipe usually has a round cross section. Further, the connecting piece is not preferably connected vertically or perpendicularly to an outer side of the wall of the exhaust pipe, but sloped in relation to the vertical direction. As a result, the geometric transition between the exhaust pipe usually provided with a round cross section and the connecting piece usually provided with a round cross section is comparatively complicated. The manufacture of such an injection device is comparatively complicated as a result. In particular, a relatively large flange is necessary at the inner end of the connecting piece in order to make it possible to weld this connecting piece on the outer side of the exhaust pipe. If, however, the connecting piece is manufactured integrally with the flange as a cast part, the connecting piece will have a relatively large thermal mass. As a result, the connecting piece can absorb a relatively large quantity of heat during the operation of the exhaust system and conduct this heat to its outer end and transmit it to the injector there. Since the injector is comparatively heat-sensitive, a large thermal mass of the connecting piece may lead to damage to or destruction of the injector.
As an alternative, it is conceivable, for example, to use a separate adapter, which has, on the one hand, the flange and is adapted to the geometry of the outer side of the exhaust pipe, and is adapted, on the other hand, to the geometry of the inner end of the connecting piece. The adapter can then be welded relatively easily to the connecting piece, on the one hand, and to the exhaust pipe, on the other hand. As a result, the connecting piece can be embodied with a markedly reduced wall thickness, which reduces the thermal mass of said connecting piece. However, this manufacturing operation is comparatively complicated because of the increased number of parts, because relatively many steps and components are necessary.